1. Field of the Invention
The present disclosure relates generally to control valves for use in freight car brake equipment, and more particularly to a valve that maintains the brake cylinder pressure in such control valves.
2. Statement of the Prior Art
Control valves used in freight car brake systems or equipment (e.g., the DB-60 control valve manufactured by New York Air Brake LLC, Watertown, N.Y. USA or the ABD/ABDW/ABDX-type control valves manufactured by Wabtec Corporation, Wilmerding, Pa. USA) are well known. If such control valves supply air pressure to the brake cylinder of a freight car, and the plumbing between the control valve and the freight car has a leak, then the brake cylinder will not maintain the original set pressure. This could be due to, for example, a leaking packing cup in the brake cylinder, a leaking hose, or any other reason that would lead to the leaking of pressure between the connection of the control valve and the brake cylinder.
One approach is described in U.S. Pat. No. 4,474,412. The freight car brake equipment for truck mounted air brakes includes an ABDW-type valve and auxiliary and emergency reservoirs with pipe connections therebetween. There is a brake pipe and a connection between the brake pipe and the ABDW valve. Truck mounted brake cylinders are located at each truck and a brake cylinder pipe, which connects the brake cylinders with the ABDW-type valve. A brake cylinder pressure maintaining valve is connected to the brake pipe, brake cylinder pipe and volume reservoir, as well as to the emergency reservoir. The brake cylinder pressure maintaining valve operates to provide air from the volume reservoir to the brake cylinder pipe and thus the brake cylinders when pressure in the brake cylinder pipe drops below a predetermined level, thus insuring a minimum brake application pressure, regardless of the piston travel of the brake cylinders.
Another approach is described in U.S. Pat. No. 4,493,511. This freight car brake equipment includes a brake cylinder pressure maintaining valve which, during brake application, takes air from the brake pipe and feeds it into the brake cylinder until it reaches minimum brake pressure. To avoid the need for an extra valve, the brake cylinder pressure maintaining valve is used as a quick service bulb exhaust valve during brake release and means are provided to ensure that the brake cylinder pressure maintaining valve opens immediately on brake release. Once the brakes have been applied, pressure in the brake cylinder is maintained from the quick service bulb. Brake cylinder exhaust pressure taken from a point after the brake cylinder exhaust valve, but before the brake cylinder exhaust choke, is directed to a chamber above the inshot and brake cylinder maintaining valve diaphragm. Since the pressure in a port upstream of the brake cylinder exhaust choke attains a pressure within about 10 psi of the chamber, a spring in another chamber can open the brake cylinder maintaining valve. Thus, immediate opening of brake cylinder maintaining valve occurs when the bi-stable operator moves to the release position. Bulb pressure from a QS bulb is then able to flow via the BC maintaining choke and fall with the fall of brake cylinder pressure.
Such prior art approaches are somewhat complicated. It would be far more desirable to provide a brake cylinder maintaining valve which is simpler to implement and operate.